06/08/2022
By Joshua Wancura
The Kennedy College of Science, Department of Physics & Applied Physics invites you to attend a Ph.D. dissertation defense by Joshua Wancura entitled “Assessing the Feasibility of Using MRI to Image Radiation Chemistry Changes Produced During Radiotherapy.”
Candidate: Joshua Wancura
Date: Thursday, June 23, 2022
Time: 2 p.m.
Location: This will be a virtual defense via Zoom. Those interested in attending should contact Joshua_Wancura@student.uml.edu to request access.
Committee:
- Atchar Sudhyadhom, Ph.D., DABR, Assistant Professor of Radiation Oncology, Harvard Medical School
- Erno Sajo, Ph.D., Professor, Department of Physics & Applied Physics, University of Massachusetts Lowell
- James Egan, Ph.D., Professor, Department of Physics & Applied Physics, University of Massachusetts Lowell
Abstract:
Experiments demonstrating that x-rays are capable of inducing MR signal changes in aqueous solutions are presented. A paramagnetic relaxation enhancement mechanism associated with radiation chemistry changes is proposed to explain this x-ray-induced signal change. Images of water and 10 mM coumarin phantoms were acquired on a 0.35-T MR-linac before, during, and after delivery of 80 Gy using an inversion-recovery dual-echo sequence with water nullified. Radiation chemistry simulations of these conditions were performed to calculate changes in oxygen and free radical concentrations. Published relaxivity values were then applied to calculate the resulting T1 change, and analytical MR signal equations were used to calculate the associated signal change. Compared to pre-irradiation reference images, water phantom images taken during and after irradiation showed little to no change, while coumarin phantom images showed a small signal loss in the irradiated region (CNR 1.0 - 2.5). Radiation chemistry simulations found oxygen depletion of -11 µM in water and -31 µM in coumarin, resulting in a T1 lengthening of 24 ms and 68 ms respectively, and a simulated CNR of 1.0 and 2.8 respectively. This change was consistent with observations in both direction and magnitude. Steady-state superoxide, hydroxyl, hydroperoxyl, and hydrogen radical concentrations were found to contribute less than 1 ms of T1 change. Conclusions of this study were that observed radiation-induced MR signal changes were dominated by an oxygen depletion mechanism, while free radicals played a minor secondary role under steady-state conditions. Future applications may include in vivo FLASH treatment verification, but this would require an MR sequence with better SNR and higher temporal resolution than the one used in this study.